This invention is an improvement of the automatic pellet producing system of U.S. Pat. No. 3,932,736 owned by the same assignee, the disclosure of which is incorporated herein by reference. As explained more fully in the prior patent, feed pellet producing systems generally include a bin for containing a source of dry feed or other material from which the pellets are to be made, a mixer/conditioner where selected amounts of heat and moisture are added to the material as it is fed from the bin, and a roll and die into which the material is fed from the mixer/conditioner where it is pressed through holes in the die sidewall by the roll and cut off by a knife to form pellets. The formulation for most feed pellets includes not only the dry feed, but moisture in the form of water or steam, and molasses. The rate of input of these various ingredients determines not only the formulation of the finished pellets but also the temperature of the mix as it is processed and the load on the electric motor which is generally used to drive the die and roller. Of course, each of the ingredients affects the load in a different way with temperature and moisture content being found to be of critical importance for high quality and quantity production of pellets.
In the prior patent, an automatic system is disclosed and claimed for measuring a .DELTA.T parameter defined as the temperature difference of the material as it moves into and out of the mixer/conditioner. This .DELTA.T parameter is used to control the operation of the pellet mill within a specified normal operating load range by controlling the flow of steam into the mixer/conditioner. As a temperature increase or decrease may be achieved by varying the amount of steam, the change in temperature is also related to the change in moisture present in the mixture. Thus, the patent discloses and claims a system for controlling pellet mill operation in response to the moisture level of the mixture by monitoring the temperature change of the material as it flows through the mixer/conditioner. The invention disclosed and claimed in the patent is a very valuable and useful invention which represents a significant improvement over the prior art available at the time of invention in controlling pelleting operations. The controller includes various other features, such as overload protection, provision for automatic start-up after overload, automatic advancement of the feed rate and steam after overload, and other features which were theretofore unavailable in the prior art.
The assignee herein is also the owner of application Ser. No. 139,841 filed Apr. 14, 1980, now U.S. Pat. No. 4,340,937, which represents an improvement over the patented system disclosed and claimed in U.S. Pat. No. 3,932,736. The disclosure of application Ser. No. 139,841, now U.S. Pat. No. 4,340,937, is incorporated herein by reference. In that application, a control system is disclosed wherein a temperature difference or .DELTA.T is taken across the mill itself (.DELTA.T mill) and the rate of input of moisture related ingredients (such as steam) is controlled to maintain .DELTA.T mill within a prescribed range; the range depending on pellet formulation. The advantages in using .DELTA.T mill instead of .DELTA.T are more completely explained in the application, but the concept is that .DELTA.T mill is a more direct measure of the work done by the mill during pelleting. For a given pellet formulation, it is thought that optimum pelleting is achieved by maintaining a certain work level in the mill, and then running mill load up near maximum to achieve maximum throughput. The .DELTA.T mill parameter may be fairly well controlled by adjusting the rate of input of moisture related ingredients; an increase in moisture lowering friction in the die and .DELTA.T mill.
Although both of these inventions represent patentable improvements over the prior art, work continues in understanding the pelleting process and automating pellet mills to achieve maximum production of the highest quality durable pellets with the least amount of energy, equipment wear, and human intervention. As pelleting is generally considered to be an art at this stage of its development and not a science, it is believed that the final solution for automatically controlling a pelleting operation to achieve optimum results for all formulations has not been found. For the time being, it may be that certain formulations utilizing different kinds of dry feed may be processed with greater efficiency in certain mills using .DELTA.T as the controlling parameter, .DELTA.T mill in others, and possibly some other parameter or combination of parameters in others. With the present state of the art, it would be desirable to have a controller which permitted operator selection of .DELTA.T, or .DELTA.T mill or some other system operating parameter as a controlled variable not only to match the particular idiosyncracies of a pellet mill, but also to adapt to different formulations in the same mill. It would also be desirable to have the capability to add other system operating parameters as controlled variables, or combine them in some way without making extensive changes to the control hardware. Also, a controller should be "human engineered" to guide an operator through a pelleting run by telling him what to do and when to do it, display needed information to assure the operator of proper mill operation, and warn him should equipment malfunction. A manual mode should also be available to accommodate special pelleting runs, to permit equipment testing, and operation of the mill under full operator control.
To solve these and other problems, applicant has succeeded in developing a microprocessor based pellet mill control which is programmable to define the relationship between any one or combination of system operating parameters and control the rate of input of any one or more ingredients to produce pellets having the desired formulation at a desired load. As the microprocessor is easily re-programmed by substituting one or more "chips", the controller may be easily modified to permit different modes of operation as the art of pelleting develops into a science. This virtually eliminates the worry of equipment obsolescence inherent with prior art controls. An operator's control panel contains the entire controller, with all the switch inputs, digital displays, and indicating lights being built right into the front thereof.
As his best mode, applicant has developed a software package which permits operator selection of either .DELTA.T, .DELTA.T mill or mill load control by simply dialing in the desired temperature differential or zero. This software package provides other unique features such as preset load test points which enable or disable input of certain ingredients and a built in time delay before a sensed change in the operating parameter results in an adjustment in ingredient input.
In a typical automatic run with the software package herein, applicant's controller will check that all necessary equipment is electrically enabled and that there is feed in the input bin. It then starts a gradual flow of feed through the system until a minimum mill load is achieved. At that point, and as the system load stabilizes, the controller will automatically enable the input of liquids and ratio them in proper proportions to match the formulation as selected by the operator. Depending on the mode of control selected, the controller will continually adjust the flow rate of liquids as required to maintain the desired temperature differential and formulation dialed in by the operator and continually increase the flow of dry feed to bring the mill load up to the value preselected by the operator. The controller will decrease or completely stop the flow of dry feed as required if the mill overloads, the rolls slip, or any of the other vital equipment becomes non-functional. If the controller senses a stoppage in flow, or the mill is stopped by the controller, the liquids are automatically disabled. At the end of the run, conditions suitable for repelleting of the "fines" are provided and after the fines are re-pelleted, the system shuts down automatically and waits for the next run to be set up by the operator.
Virtually instantaneous monitoring and control of ingredient input enable applicant's microprocessor based controller to operate the mill at levels much nearer optimum than in a manual mode relying on human attentiveness and much slower reaction times. It is believed that maximum throughput is generally attained at conditions near plugging, where the mass of material would otherwise clog the mill causing overload of the mill drive motor. If plugging occurs, the mill must be shut down, cleaned out, and restarted which results in an expensive waste of material and a great loss of time. No human can approach a microprocessor in speed and attention span. Thus, with applicant's controller, safe and efficient operation at levels much closer to optimum are feasible, resulting in greater throughput with less energy, time, and equipment wear.
Some of the benefits and features of applicant's invention have been described. A fuller understanding thereof may be attained by referring to the drawings and description of the preferred embodiment which folows.